1. Field of the Invention
The invention relates to a surface-mountable light-emitting diode (SMD) structural element in which an optoelectronic chip is attached to a chip carrier part of a lead frame via a heat-conducting connection. The lead frame has a connecting part disposed at a distance from the chip carrier part which is connected with an electrically conducting electrical contact of the optoelectronic chip. The optoelectronic chip and a part of the lead frame are encased by a casing forming a foundation, whereby the foundation has a first main surface and a second main surface opposite the first main surface. The external connections of the chip carrier part and the connection part provided in the lead frame project outside the casing, whereby the external connections are either bent outside the foundation at first to the second main surface of the foundation opposite the first main surface and in the further course either below the foundation to its center or are bent away to form rocker-shaped connection stumps. It particularly relates to an optoelectronic structural element with an optoelectronic semiconductor chip, especially with a light-diode chip (LED) operated at a high output.
Such a structural element is known, for example from Published, European Patent Application EP 0 400 176 A. A so-called TOPLED is described here, in which a light-emitting semiconductor chip (LED chip) is attached to a flat chip carrier part of a lead frame. The lead frame consists of a chip carrier part with external connections and a connection part disposed at a distance from the latter, thus electrically insulated from it. The chip carrier part with the semiconductor chip, the connection part, and partial areas of the external connections are encased by a casing consisting of an emission-impermeable foundation with a recess and an emission-permeable window part filling this recess. The chip carrier part and the connection part are encased by the foundation or are embedded in it so that partial areas of upper sides of the chip carrier part and the connection part with the remaining floor surface tightly close the recess. The semiconductor chip is completely encased by the emission-permeable window part except for its lower side, with which it lies on the chip carrier part. The recess and its inner surfaces are formed and disposed so that they form an essentially truncated cone-shaped reflector for the radiation emitted by the semiconductor chip.
An optoelectronic structural element is described in Published, Non-Prosecuted German Patent Application DE 195 36 454 A1 in which, to improve the heat conduction of the semiconductor chip, two external connections of the chip carrier part of the lead frame are widened opposite the external connections of the connection part.
In the case of the known optoelectronic structural elements described above, the semiconductor chip operates at high currents and thereby high output, as for example is the case with so-called power-LEDs, leading to high heating because of insufficient heat conduction from the semiconductor chip. This heating frequently leads to impairments of the functional ability of the semiconductor chip, such as accelerated aging, breaking off of the semiconductor chip from the lead frame, breakage of the semiconductor chip away from the lead frame, breaking away of bond wires, or destruction of the chip. The known widened external connections of the chip carrier part favor delamination of plastic from the lead frame that, for example, can cause penetration of moisture to the semiconductor chip.
It is accordingly an object of the invention to provide a surface-mountable light-emitting diode structural element that overcomes the above-mentioned disadvantages of the prior art devices of this general type, in which improved heat conduction from the chip is assured, without at the same time substantially altering the housing dimensions and without at the same time substantially increasing delamination danger.
With the foregoing and other objects in view there is provided, in accordance with the invention, a surface-mountable light-emitting diode (SMD) structural element that is formed of a lead frame having a chip carrier part, three separate external connections, and a connection part disposed at a distance from the chip carrier part. The three separate external connections extend outward in three different directions, starting from the chip carrier part. A light-emitting semiconductor chip is heat-conductively connected to the chip carrier part of the lead frame. The light-emitting semiconductor chip has an electrical contact electrically conductively connected to the connection part. A casing is provided and has a foundation encasing the chip carrier part, the connection part and partial areas of the three separate external connections. The foundation has a first main surface, a recess formed therein, and an outward facing second main surface disposed opposite the first main surface. The three separate external connections and the connection part project outside of the casing. The three separate external connections and the connection part are bent outside of the foundation toward the outward-facing second main surface of the foundation and in a further course are further bent below the foundation toward a center of the outward-facing second main surface or away from the foundation for forming rocker-shaped connection stumps. The three separate external connections project from the casing on at least two sides of the casing at different places at a distance from each other. The heat-conducting connections as seen in a top view of the lead frame project from the casing on at least two sides and starting from the chip carrier part run toward the outside in a stellate form within the casing and separately from each other. The light-emitting semiconductor chip is disposed in the recess.
In the structural element according to the invention, it is provided for the chip carrier part to have at least three separate thermally conducting external connections connected with the chip carrier part which project from the casing at different places at a distance from each other and are configured so that they all simultaneously lie on the connection or lead plate in the assembled state of the structural element provided for mounting the structural element. The heat resulting in the chip in the operation of the structural elements is consequently fed into the lead plate at three different points and is distributed on a broad surface of the latter. A distinctly improved heat conduction from the optoelectronic chip is thereby obtained.
In a particularly preferred form of the structural element according to the invention, the external connections of the chip carrier part, in a top view of the lead frame, run separately from each other in an essentially stellate form, starting from the chip carrier part. The heat conduction points from the structural element to the lead plate thereby present large distances from each other, whereby a very large-surface distribution of the thermal energy derived from the chip in the operation of the structural element is conducted away by way of the chip carrier part and its external connections.
The external connections in the area in which they run outward in a star shape advantageously present longitudinal central axes, wherein two adjacent connections always present an angle of about 90xc2x0 to each other. In this configuration, the plastic area between the connections is maximum, whereby the delamination danger is reduced, for example in the case of temperature fluctuations.
If the structural element is provided with at least a first and a second optoelectronic chip, the chip carrier part according to the invention presents at least two external connections which project from the casing at various places on it with a distance between them. The lead frame here presents at least two connection parts with an external connection in each case, which also project sideways from the casing. It is also preferable here for the external connections of the chip carrier part and the connection parts, viewed from the top of the lead frame, to essentially be disposed in the stellate form, whereby a maximum interval of the external connections of the chip carrier part from each other is assured. The thermal energy conducted from the chip in the operation of the structural element by way of the chip carrier part and its external connections is thereby fed into the lead plate at points located relatively far from each other, so that here, too, a very good heat distribution on the lead plate is obtained.
In the case of the last-mentioned structural element, it is especially advantageous for the external connections of the chip carrier part, viewed from a top view of the lead frame, to be disposed displaced diagonally to each other. They project from the chip carrier part on side surfaces opposite each other of the casing having a preferably essentially square shape. The connection parts are thereby disposed on different sides of the chip carrier part, and their external connections likewise project on side surfaces of the casing opposite each other. From a view of the lead frame, concerning the chip carrier part, they are disposed displaced diagonally to each other in relation to the external connections of the chip carrier part.
In order to further improve heat conduction from the chips, if necessary, the chip carrier part can present more than two external connections, which again project from the casing at various places at a distance from each other. The number of the external connections can be increased further as required according to the permissible structure size of the structural element.
In the case of the structural element in accordance with the invention which can be attached to the lead plate so that, in its operation, the radiation emitted by the chip is radiated essentially parallel to the lead plate (laterally emitting structural element), the chip carrier part exhibits at least two external connections and the connection part at least one external connection which project separately from each other at one and the same side surface of the casing. The external connection of the connection part is thereby preferably disposed between the two external connections of the chip carrier part.
In an especially preferred further development of the last-mentioned structural element, the external connection of the connection part is narrower than the two external connections of the chip carrier part. Of course, the external connections of the chip carrier part can optionally also be wider than the external connections of the connection parts.
In a further development of the above mentioned side-emitting structural element, the chip carrier part is additionally provided with greater heat conduction via at least one heat-conducting cooling fin, which projects from a side surface of the casing other than from where the connection parts project. The cooling fin alone or a further cooling device thermally attached to the cooling fin assures a still further improved heat conduction from the chip.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a surface-mountable light-emitting diode structural element, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.